1. Field of the Invention
The present invention relates to a sheet cassette for use in an image forming apparatus. Particularly, the invention relates to a sheet cassette which is capable of containing inch-based size sheets as well as centimeter-based size sheets, and to an image forming apparatus including the sheet cassette.
2. Description of Related Art
In an image forming apparatus, a sheet is fed from a sheet cassette, and an image is formed on the sheet. The sheet cassette includes a width limiting plate which is slidable widthwise of sheets contained in the cassette according to the width of the sheets (as measured perpendicularly to a direction in which the sheets are fed out of the sheet cassette). By preliminarily positioning the width limiting plate in the sheet cassette according to the size of the sheets, the sheets can be aligned widthwise in a predetermined position in the sheet cassette. Thus, a sheet fed out of the sheet cassette is properly transported, and an image is formed on the sheet in the image forming apparatus.
The sheet cassette includes a rail for guiding the width limiting plate in the movement direction of the width limiting plate, and recesses are provided at positions corresponding to various sheet sizes on an edge of the rail. The width limiting plate has a claw engageable with the recesses. The width limiting plate can be fixed at a position corresponding to the size of the sheets with the claw thereof in engagement with a predetermined one of the recesses.
Meanwhile, sheets to be used in the image forming apparatus include inch-based size sheets and centimeter-based size sheets, which are often selectively contained in a single sheet cassette. In this case, recesses for the widths of the inch-based size sheets (hereinafter referred to as “inch-based size engagement recesses”) and recesses for the widths of the centimeter-based size sheets (hereinafter referred to as “centimeter-based size engagement recesses”) are provided on the edge of the rail. See, for example, Japanese Unexamined Patent Publication No. 2004-256274.
A user of the image forming apparatus properly positions the width limiting plate with the claw in engagement with the predetermined recess, whereby the centimeter-based size sheets or the inch-based size sheets can be set in a proper position in the sheet cassette.
In the prior art, however, the inch-based size engagement recesses and the centimeter-based size engagement recesses are provided in very close relation on the edge of the rail. For example, a so-called letter size based on inch is 216 mm×279 mm, and a Japanese Industrial Standard Column-A No. 4 size (A4 size) based on centimeter is 210 mm×297 mm. That is, a difference in short edge length between the letter size and the A4 size is 6 mm. In a sheet cassette adapted to limit the position of the sheets by sliding a pair of width limiting plates symmetrically widthwise, inch-based size engagement recesses for the letter size are spaced only 3 mm from centimeter-based size engagement recesses for the A4 size on the edge of the rail.
Hence, there is a possibility that the user of the image forming apparatus sets the claw in engagement with a wrong recess. In this case, the sheets cannot be aligned widthwise in the proper position in the sheet cassette by the width limiting plate. Therefore, a sheet fed out of the sheet cassette is offset widthwise from a predetermined position when transported through a sheet transport path in the image forming apparatus.
Accordingly, an image formed on the sheet will be offset widthwise and, in the worse case, the sheet is jammed in the sheet transport path in the image forming apparatus.